Soft x-ray absorption spectroscopy study of electrochemically formed passive layers on AISI 304 and 316L stainless steels

The structure and composition of passive films electrochemically formed on AISI 304 and 316L stainless steels in a chloride-containing solution have been studied by soft x-ray absorption spectroscopy. Soft x-ray absorption spectra were taken at the oxygen 1s edge and at the transition metals (Cr, Fe, Ni) 2p edges, making it possible to determine the main contributions to the passive film's composition. The soft x-ray absorption spectra at the Cr 2p edges indicate that in all cases the passive film is mainly formed by Cr₂O₃. Spectra at the Fe and Ni 2p edges exhibit no significant contribution of Ni and Fe oxides to the passive layer composition. However, differences in spectral shape with respect to metallic Ni and Fe suggest the presence of a small amount of hydroxides, which is maximum for the AISI 304 stainless steel polarized at the lowest scan rate. The spectra at the O 1s edges confirm the results obtained at the transition metal edges.